1. Field of the Invention
The present invention relates generally to seismic signal processing and, more particularly, to apparatus and methods for improved interpretation of AVO data from seismic data comprising techniques for obtaining additional information from AVO data to more reliably and consistently detect and/or visualize formations of interest and attributes thereof.
2. Description of the Background
Seismic data is produced in response to a seismic source which generates sound waves that reflect from subsurface formations. The sound waves are detected by a geophone array and recorded. Seismic data is acquired with a geometry that results in the same subsurface points being sampled by numerous source to receiver pairs with different angular relationships. The source to receiver pairs are often described in terms of their offsets with respect to a common midpoint. The processing of seismic data results in, among other things, a gathering of all common midpoint source to receiver pairs. The set of gathered traces may be called the CMP or CDP gathers or, simply the gathers.
FIG. 1 shows the process of AVO gradient analysis to obtain AVO data. AVO (amplitude versus offset) analysis is well known in the art of seismic signal processing. Seismic gather 10 is plotted in terms of time versus offset. At any particular time t (or depth), an amplitude versus offset plot can be calculated as indicated at 12. The value of the amplitude of each sample on each trace in the set of gathers is plotted against a measure of the offset of the trace from the CMP (generally the sin2 or angle of incidence). A best fit line 14 yields a Y-intercept and a slope or gradient (G). It will be understood that other names of the variables involved may be utilized, although P and G are commonly used variable names.
All samples of all CDPs may be analyzed yielding P and G sections or 3-D volumes. In this framework, time and/or depth may be taken to represent a z-axis location, and geographic coordinates represent x and y axes or locations to provide an (x, y, z) framework. It will be understood that x, y, z coordinates can be used to describe 1-D, 2-D, 3-D data volumes, as well as 4-D (sometimes referred to as time-lapse) data. As an example, all the P values may be plotted in a view representative of a section or slice of the Earth. As well, it will be understood that various types of coordinate systems may be utilized so that an (x, y, z) framework, which may effectively be a Cartesian coordinate system, could be transposed into other types of coordinate systems, such as spherical, cylindrical, polar, or the like and are therefore essentially equivalent for purposes of the present invention.
A representative example of a sectional view of a slice of the Earth in an (x, y, z) framework, although not P values, is shown in FIG. 4B, which is discussed hereinafter. The G values may be plotted in another section or slice of the Earth. A full stack of all values may also be plotted in another section or slice. Looking at these different sections or slices, one of skill of the art may ask where are the sands? Gas? What are the porosities? What types of rocks are these? It may appear that each different section or slice made by plotting these different values seems to give or appear to give a different answer to these questions.
Due the problem of the different possible answers to these relevant questions based on the above described analysis, various efforts have been made to improve the quality of answers obtainable. Some of these efforts are described below in a listing of background U.S. Patents. One prior art method is shown in U.S. Pat. No. 5,440,525, which is also discussed below. In this approach, the AVO data is plotted as discussed above, and a background trend line is determined. The distance from the background trend line is then considered to provide an indication of hydrocarbons. However, the results of this technique can overlook what are considered by the inventors to be significant information that may result in overlooking important pay zones.
U.S. Pat. No. 5,297,108, issued to Swan on Mar. 22, 1994, entitled “Seismic Velocity Estimation Method,” and U.S. Pat. No. 5,258,960, issued to Swan on Nov. 2, 1993, entitled “Seismic Velocity Estimation Method”, disclose a method for detecting errors in estimated seismic velocities used in a normal moveout correction of a gather of traces selected from conventional, common midpoint seismic data. Zero offset reflectivity and amplitude versus offset slope traces are derived from the NMO corrected gather. Analytic traces are calculated for the zero offset reflectivity and amplitude versus offset slope traces. The analytic zero offset reflectivity trace is multiplied by the complex conjugate of the analytic slope trace and the imaginary part of the product indicates estimated velocity errors. The velocity error indicator is used to correct the velocity estimates so that the normal moveout process may be reperformed without the errors caused by incorrect velocity estimates. Alternatively, the velocity error indicator itself is plotted on a seismic section as an indicator of characteristics of subsurface earth formations.
U.S. Pat. No. 5,440,525, issued to Dey-Sarkar et al on Aug. 8, 1995, entitled “Seismic Data Hydrocarbon Indicator,” discloses a method for displaying seismic data to provide direct indications of the presence of hydrocarbons. Seismic data is processed using conventional amplitude versus offset techniques to obtain zero offset reflectivity, or A, traces and the amplitude versus offset slope, or B, traces. AB cross plots of each trace are then generated. Each sample point on the cross plot is then assigned a value corresponding to its deviation from the regression line of the cross plotted AB points. The assigned values are then plotted in their corresponding time sample positions to generate a trace or display providing a direct indication of hydrocarbons.
U.S. Pat. No. 5,515,335, issued to Swan on May 7, 1996, entitled “Seismic Trace Overburden Correction Method”, discloses a method for generating improved displays of seismic data by processing seismic amplitude versus offset data to correct for overburden effects. Analytic traces are calculated for the zero offset reflectivity, A, trace and the amplitude versus offset slope, B, trace of the AVO data. Statistics for the A and B traces within a selected window in time and common depth point space about a selected sample point are calculated. The statistics include root mean square amplitudes of the A and B traces and the correlation coefficient. Desired statistics are selected and used with the measured statistics to correct the A and B traces.
U.S. Pat. No. 5,661,697, issued to Swan et al on Aug. 26, 1997, entitled “Method and Apparatus for Detection of Sand Formations in Amplitude-Versus-Offset Seismic Surveys”, discloses a method and apparatus for analyzing amplitude-versus-offset (AVO) seismic data to distinguish sand formations, such as Morrow sands, from limestones and other similar intervals. For each of the traces in the survey, AVO intercept and AVO slope traces are generated, preferably after normalization of the amplitudes of the traces to account for geophone coupling variations. After normalization and conventional processing and corrections, spatial summation may be performed to further improve the traces. AVO trend lines are then generated, preferably on a weighted window basis, to generate localized trend lines against which the intercept and slope values of individual depth points may be compared. This comparison allows the plotting of AVO intercept versus AVO slope deviation from the trend line, from which sand formation interfaces may be identified by their presence in certain quadrants of the intercept-slope deviation cross-plot.
U.S. Pat. No. 5,784,334, issued to Sena et al on Jul. 21, 1998, entitled “Method and System for Detecting Hydrocarbon Reservoirs Using Amplitude Versus Offset Analysis of Seismic Signals”, discloses a computer-operated method for analyzing seismic data to discern the presence of hydrocarbon-bearing formations. According to the disclosed method and system, amplitude-versus-offset (AVO) analysis is performed to assign, for each depth point in a survey region, an AVO intercept value and an AVO gradient value; the AVO intercept value corresponds to the zero-offset response for acoustic reflections from the depth point, while the AVO gradient value corresponds to the rate of change of the amplitude as a function of the angle of incidence of the acoustic energy (typically, with the square of the sine of the angle). An AVO indicator indicative of the presence of hydrocarbons at a subsurface stratum is derived to correspond to the rate of change of the product of the AVO intercept value and the AVO gradient value for the depth point under analysis, along the direction of a deviation vector of the AVO intercept value and the AVO gradient value from a background trend for depth points surrounding the depth point under analysis in time and space. The background trend, and thus the deviations, may be a straight line in a space having AVO intercept value and AVO gradient value as axes, or may be a statistical trend used in deriving the deviations. Use of the disclosed method and system has been observed to indicate deep hydrocarbon-bearing formations that are not detectable using conventional AVO analysis.
U.S. Pat. No. 6,058,074, issued to Swan et al on May 2, 2000, entitled “Method and System for Detecting Hydrocarbon Reservoirs Using Amplitude-Versus-Offset Analysis with Improved Measurement of Background Statistics”, discloses a computer system and method of operating the same to apply overburden corrections to seismic signals prior to amplitude-versus-offset (AVO) analysis. The system and method retrieve common midpoint gathers of the seismic signals, and generate analytical, or complex, AVO intercept and AVO slope traces therefrom, effectively stacking the traces in each gather. Over a sliding time window of the stacks, the computer system generates p-measure standard deviation and correlation statistics, preferably using a p-measure value less than one. The AVO intercept and AVO slope traces are then modified, at each depth point of interest corresponding to a time window placement, according to the relationship between the p-measure statistics and the desired statistics for the background distribution. Conformance of the background statistics to known values can be achieved, thus eliminating offset-dependent contamination of the AVO data; this is accomplished with minimal influence from AVO anomalous points, improving the sensitivity and accuracy with which petrophysically-interesting strata may be detected from AVO traces.
The article “A Comprehensive AVO Classification,” by the present inventors, was published October 2003, and discloses an AVO classification wherein all possible combinations of normal-incidence reflectivity and offset-dependent reflectivity, for all seismic energy coming from the top of nonshale lithologies, are subdivided into 10 types. Broadly, the divisions result from dividing a unit circle into eight domains depending on the attributes of P and G: They each may be positive, near zero, or negative, independently of the other. Two of the domains are further subdivided and all of the domains are grouped by a further distinction as to whether the types are: conforming, where decreasing shaliness and increasing gas have similar effects: or, nonconforming, where decreasing shaliness and increasing gas have opposite effects (or gas has no effect). The definitions are divorced from a gas-sand association and applied, without discrimination, to any nonshale lithology. This approach also departs from conventional AVO analyses in that the AVO attribute becomes a continuously distributed attribute of the seismic data.
The above cited art does not overcome the disadvantages discussed hereinbefore. Consequently, there remains a long felt need for improved methods of seismic signal analysis for more accurately locating formations of interest. Those skilled in the art have long sought and will appreciate the present invention which addresses these and other problems.